The present invention relates to image reproduction systems, and more particularly to a laser driver for fan injection laser diode and method of operating a laser driver in a high resolution laser recorder.
Laser image recorders may use a semiconductor laser diode (referred to herein as an injection laser diode, "ILD") to reproduce an image on a photographic media. The image is typically provided to the recorder in the form of digital or analog data that is used to modulate the ILD output to expose the photographic media. Due to the nature of the data and image, the operation of the device is complex. For example, recording resolution may be on the order of 300 pixels per inch, yielding nearly eight million pixels for an eight inch by eleven inch image. The image may be reproduced in about a half a minute (over 264,000 ILD operations per second) and may have 256 shades of gray with eight bits per pixel to describe the shading. Laser image recorders find application in a variety of fields, such as reconnaissance, medical (e.g., X-ray image reproduction), satellite imagery, computer-generated graphics, and the like.
The image reproduction process involves numerous steps in which an error in one step may be magnified in the next, and one of the more vexing problems encountered throughout the process is that of the effect of temperature on ILD operation. While the present invention addresses several problems, it may be more clearly understood by considering the invention as it addresses the temperature problems at several steps.
Baseline Drift.
Consider first the affect of temperature on the stability of the signal from the laser driver that controls ILD output. The relative range of operation of a laser driver for an ILD may be from 1 to 4000, and throughout this range the ILD desirably maintains a precise transfer function between the electrical input signal Vin (the information-bearing portion of the input), and the optical output power, Pout: EQU Pout=mVin+b (1)
where, m is the gain of the laser driver, and b is the power offset of the laser driver. That is, the laser driver signal for controlling ILD output may be viewed as having a variable (modulated) information-bearing portion, mVin, and a fixed power offset (baseline) portion, b.
The stability of the offset b is highly desirable to the maintenance of a precise transfer function. However, changes in temperature (as well as other factors such as aging) cause the power offset b to drift (offset drift is also denoted baseline drift). The temperature changes that may cause baseline drift may be felt in various components and attempts to maintain the temperature of all the components have not proven workable in many applications. Drift control measures that rely on feedback have proven more successful. However, feedback methods for wide bandwidth, wide dynamic range laser drivers require a large number of components with very high stability.
ILD Protection.
Temperature may also affect ILD operation more directly because an ILD will be destroyed if it is provided with a current that exceeds a maximum value. Significantly, the allowable maximum current value changes with temperature. By way of further explanation, and with reference to FIG. 1, an ILD operating at the intersection of lines A and B (25.degree. C., curve 2) may be provided with the forward current shown (the total of variable and fixed portions) and have the optical output power shown. If the forward current remains fixed, but the ILD temperature drops to 0.degree. C. (curve 1) the ILD will have exceeded its maximum output and be destroyed. On the other hand, if the ILD temperature rises to 50.degree. C. (curve 3) and the current remains on line A, the ILD's output will be reduced. The temperature curves such as shown in FIG. 1 are inherent characteristics of ILDs.
Several methods have been employed to protect the ILDs. One is to limit the amount of current available to drive the ILD by inserting a large ohmage resistor between the ILD and the available power source (voltage). The available current is thereby limited to: EQU I=(V-Vd)/R (2)
where V is available voltage, Vd is ILD "on" voltage, and R is the ohmage of the limiting resistor. In this approach, the problem of temperature induced variations of maximum current is resolved by setting the maximum available current (by selection of R) at a value less than a value that would destroy the ILD at anticipated operating temperatures. This, of course, limits the range of operation of the ILD. Further, the resistor dissipates power, and the large ohmage of the resistor limits the bandwidth of ILD modulation.
In another method of protecting the ILD, the optical output power of the ILD is monitored and a corrective feedback signal is generated to lower or stop the ILD drive signal when a predetermined maximum power has been exceeded. However, a significant amount of time is needed to determine whether the maximum power has been exceeded, to feed a signal back to the laser driver, and for the laser driver to take corrective action. This time delay limits the bandwidth and the dynamic range over which the ILD can be used.
Accordingly, it is an object of the present invention to provide a novel device and method of operating a laser driver in which the baseline drift problem is obviated by reducing the number of laser driver components that control the baseline temperature drift.
It is a further object of the present invention to provide a novel device and method of stabilizing baseline drift in a laser driver in which a test of baseline drift is made during an interval of laser driver operation when information-bearing signals are not being processed.
It is yet a further object of the present invention to provide a novel device and method of stabilizing baseline drift in a laser driver in which a predetermined reference signal is provided when it will not interfere with normal operation, and the ILD output generated by the reference signal is compared to an expected output to provide a corrective feedback to the laser driver.
It is still a further object of the present invention to provide a novel device and method of stabilizing baseline drift in a laser driver in which each comparison of an ILD output from a predetermined reference signal to an expected output is provided from a one bit converter.
It is also a further object of the present invention to provide a novel device and method of stabilizing baseline drift in a laser driver in which plural one-bit results of comparisons of ILD outputs to expected outputs are averaged to reduce the noise of the laser driver.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.